1. Field of the Invention
The present invention relates to a belt driving control apparatus for controlling the driving of a belt installed on a plurality of supporting rotating bodies, a belt apparatus using this belt driving control apparatus, and an image forming apparatus using this belt apparatus.
2. Description of the Related Art
Conventionally, image forming apparatuses using belts such as photosensitive belts, intermediate transfer belts, paper conveyor belts and the like have been seen as such apparatuses using belts. In such image forming apparatuses, high-precision driving control of the belt is essential for obtaining high-quality images. Especially in the case of tandem type image forming apparatuses using a direct transfer system which is superior in terms of image formation speed and which is suitable for achieving a compact size, high-precision driving control of the conveyor belt that conveys the recording paper constituting the recording material is required. In this image forming apparatus, the recording paper is conveyed using a conveyor belt, and is successively caused to pass through a plurality of image forming units forming images of different single colors that are disposed along the conveying direction. As a result, color images can be obtained by superimposing respective monochromatic images on the recording paper.
In such a tandem type image forming apparatus using a direct transfer system, color deviation occurs if the speed at which the recording paper moves, i.e., the movement speed of the conveyor belt, is not maintained at a constant speed. This color deviation occurs as a result of a relative shift in the transfer positions of the respective monochromatic images that are superimposed on the recording paper. When such color deviation occurs, for example, line images formed by the superimposition of images of a plurality of colors appear blurred, and white dropout occurs around the outlines of black character images formed in background images that are formed by the superimposition of images of a plurality of colors.
Furthermore, not only in such tandem type image forming apparatuses, but also in image forming apparatuses using belts as recording material conveying members that convey recording materials, or as image carrying bodies such as intermediate transfer bodies or photosensitive bodies that carry images that are transferred onto the recording material, banding occurs if the speed of movement of the belt is not maintained at a constant speed. This banding is an irregularity in image density that occurs as a result of the belt movement speed being accelerated or slowed during image transfer. Specifically, portions of images that are transferred when the belt movement speed is relatively rapid assume a shape that is stretched out in the circumferential direction of the belt from the original image shape; conversely, portions of images that are transferred when the belt movement speed is relatively slow assume a shape that is contracted in the circumferential direction of the belt from the original image shape. Consequently, the image portions that are stretched out show a decrease in density, while the image portions that are contracted show an increase in density. As a result, an irregularity in image density is generated in the circumferential direction of the belt, so that banding occurs. This banding is conspicuously sensed by the human eye in cases where light monochromatic images are formed.
The movement speed of the belt fluctuates for various reasons; among the causes of such fluctuation is irregularity in the belt thickness in the circumferential direction of the belt in the case of single-layer belts. For example, this irregularity in the thickness of the belt occurs as a result of a bias in the thickness of the belt along the circumferential direction of the belt seen in belts that are manufactured by a centrifugal firing system using a cylindrical mold. If such irregularity in the belt thickness is present in a belt, the belt movement speed is accelerated when portions of the belt with a large thickness are wound on the driving rollers that drive the belt; conversely, the belt movement speed is slowed when portions of the belt that have a small thickness are wound on these rollers. Accordingly, a fluctuation occurs in the belt movement speed.
The belt movement speed is determined by the distance from the surfaces of the rollers to the belt pitch line, i.e., the pitch line distance (hereafter referred to as the “PLD”). In cases where the belt is a single-layer belt made of a uniform belt-material, and the absolute values of expansion and contraction on the side of the inner circumferential surface of the belt and the side of the outer circumferential surface of the belt substantially coincide, this PLD corresponds to the distance between the center of the belt in the direction of belt thickness and the inner circumferential surface of the belt, i.e., the surfaces of the rollers. Accordingly, in the case of a single-layer belt, since the relationship between the PLD and the belt thickness is substantially fixed, the belt movement speed can also be determined by fluctuations in the belt thickness. However, in the case of belts comprising a plurality of layers or the like, as a result of mutual differences in expansion and contraction between hard layers and soft layers, the distance between the roller surfaces and a position that is shifted from the center of the belt in the direction of thickness is the PLD.
When this PLD fluctuates in the circumferential direction of the belt, the belt movement speed or belt movement distance with respect to the rotational angular speed or rotational angular displacement of the driving rollers, or the rotational angular speed or rotational angular displacement of the driven rollers with respect to the belt movement speed or belt movement distance, fluctuates. Accordingly, the belt cannot be driven at the desired movement speed.
The image forming apparatuses described in Japanese Patent Application Laid-Open No. 2000-310897, Japanese Patent No. 3,186,610 and the like may be cited as examples of apparatus which make it possible to perform driving control of the belt with such fluctuations in the PLD taken into account.
In this Japanese Patent Application Laid-Open No. 2000-310897, an image forming apparatus is disclosed in which the thickness profile (belt thickness irregularity) over the entire circumference of the belt is measured beforehand in the manufacturing process before the belt (formed by the centrifugal forming method in which fluctuation in the PLD tends to occur in the form of a sine wave over the circumference of the belt) is installed in the apparatus main body, and this data is stored in a flash ROM. In this image forming apparatus, a reference mark constituting a home position which is a reference position that is used to align the phase of the thickness profile data for the entire circumference and the actual irregularity in the belt thickness is formed, and belt driving control is performed by detecting this position as a reference so that fluctuation in the belt movement speed caused by fluctuation in the belt thickness is canceled. However, in this image forming apparatus, since the irregularity in the belt thickness is used without using the fluctuation in the PLD, accurate belt driving control is possible in the case of a single-layer belt; however, accurate belt driving control is not possible in the case of a multi-layer belt.
Furthermore, in the abovementioned Japanese Patent No. 3186610, an image forming apparatus is disclosed in which periodic fluctuations in the belt movement speed are detected by forming a detection pattern on the belt, and detecting this pattern with a detection sensor. In this image forming apparatus, the rotational speed of the driving rollers is controlled so that the detected periodic fluctuations in the belt movement speed are canceled.
However, in the image forming apparatus described in the abovementioned Japanese Patent Application Laid-Open No. 2000-310897, a measurement process that measures the irregularity in the belt thickness is required in the belt manufacturing stage; furthermore, a high-precision belt thickness measuring instrument must be used in this measurement process. Accordingly, the problem of a greatly increased manufacturing cost arises. Furthermore, the following problem also arises: namely, when the belt is replaced with a new belt, the work of inputting thickness profile data peculiar to this new belt into the apparatus is required.
Furthermore, in the image forming apparatus described in the abovementioned Japanese Patent No. 3,186,610, it is necessary to form a detection pattern for the detection of at least one circuit of the belt in order to detect the fluctuation in the belt movement speed. As a result, the following problem arises: namely, it is necessary to consume a large amount of toner in order to form this detection pattern. In particular, in cases where the mean value of belt movement speed fluctuation information for a plurality of circuits of the belt is grasped as the fluctuation in the belt movement speed in order to detect such fluctuations in the belt movement speed with a higher degree of precision, it is necessary to form a detection pattern for a plurality of circuits of the belt, so that the problem of toner consumption becomes more serious.
Furthermore, a belt driving control apparatus that can solve these problems has been proposed in Japanese Patent Application No. 2002-230537. In this belt driving control apparatus, the rotational angular displacement or rotational angular speed of driven supporting rotating bodies is detected, and an alternating current component of the rotational angular speed of the driven supporting rotating bodies which has a frequency corresponding to the periodic thickness fluctuation in the circumferential direction of the belt is extracted from this detection data. The amplitude and phase of this extracted alternating current component correspond to the amplitude and phase of the periodic thickness fluctuation in the circumferential direction of the belt. Accordingly, on the basis of the amplitude and phase of this alternating current component, control is performed so that the rotational angular speed of the driving supporting rotating bodies is lowered at a timing at which thick portions of the belt contact the driving supporting rotating bodies, and conversely, so that the rotational angular speed of the driving supporting rotating bodies is increased at a timing at which thin portions of the belt contact the driving supporting rotating bodies. If this method is used, the belt can be driven at a desired movement speed without being affected by thickness fluctuations in the circumferential direction of the belt. Furthermore, since there is no need for a measurement process that measures the irregularity in the thickness of the belt in the belt manufacturing stage, there is no increase in the manufacturing cost as there is in the apparatus of the abovementioned Japanese Patent Application Laid-Open No. 2000-310897. Furthermore, there is likewise no need for an operation that inputs thickness profile data into the apparatus whenever the belt is replaced with a new belt, as there is in the apparatus of the abovementioned Japanese Patent No. 3,186,610. In addition, since there is no need to form a detection pattern, there is likewise no consumption of toner for the purpose of belt driving control.
However, in the case of the belt driving control apparatus proposed in the abovementioned Japanese Patent Application No. 2002-230537, since the belt thickness fluctuation approaches the periodic function of a sin function (cos function), the following inconvenience arises: namely, it is necessary to predict how the belt thickness fluctuation will occur over one circuit of the belt. Specifically, the following problem arises: namely, it is necessary to predict whether the frequency component contained in the belt thickness fluctuation is only the fundamental frequency component with the same period as the period required for the belt to complete one revolution, or whether higher harmonic frequency components are also contained in this frequency component. In most cases, furthermore, the joint portions or the like of seamed belts that have a joint seam are thicker than other portions of the belt, so that a belt thickness fluctuation may be generated in the partially protruding portions. Accordingly, the following problem is encountered: namely, it is difficult to approximate such belt thickness fluctuations with a periodic function, so that a manufacturing error is contained in such portions.